2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and related compounds elicit a broad spectrum of biochemical and toxic effects in a number of in vitro and animal models as well as humans. Many of these effects are due to the disruption of gene expression mediated by the aryl hydrocarbon receptor (AHR), a ligand-dependent basic helix-loop-helix PAS DNA-binding transcription factor. Although the effects of toxic ligands correlate well with their binding affinity for the AHR, there are an increasing number of synthetic chemicals, natural dietary products, and endogenous substances that also exhibit high binding affinity but do not elicit toxicity. The prevailing dogma suggests that the difference between toxic and non-toxic AHR ligands is due to greater ligand binding affinity and to the non-metabolizable persistent nature of toxic chemicals. Although the importance of these factors is not disputed, preliminary data and analogies to nuclear receptors indicate that ligand-dependent AHR modulation of gene expression also plays an integral role in the toxic effects elicited by TCDD and related compounds. This proposal will further examine the mechanistic differences between toxic and non-toxic ligands using toxicological, molecular, genomic and bioinformatic approaches. Specifically, the similarities and differences in global gene expression profiles elicited by two toxic AHR ligands, TCDD and 3,3',4,4',5-pentachlorobiphenyl (PCB126), and two non-toxic ligands, (-naphthoflavone (BNF) and indolo[3,2-b]-carbazole (ICZ), will be examined in vitro and in vivo using human, mouse and rat models. We hypothesize that toxic AHR Iigands elicit unique changes in gene expression compared to non-toxic ligands, due to ligand structure-dependent modulation of dioxin response element (DRE)-regulated gene expression. Statistically rigorous approaches will be used to identify significant changes in global gene expression elicited AHR ligands, and to correlate these changes to toxicity in order to identify causal relationships. Results from these studies will elucidate the mechanisms of action of toxic AHR ligands following receptor binding by correlating changes in gene expression with toxic effects. This proposal will also address concerns relevant to risk assessment, including an evaluation of the validity of extrapolating mechanisms from in vitro models to whole animals, a comparison of global gene expression responses across species, and the appropriateness of using rodent data m predict human risk. [unreadable] [unreadable] [unreadable] [unreadable] [unreadable]